Molecular theories of confined fluids

Density profiles and normal pressures predicted by three different approximate density functional free energy theories and the Fischer–Methfessel approximation to the Yvon–Born–Green (YBG) equation are compared with computer simulation results for fluids confined between planar walls. All models require as input a homogeneous fluid equation of state. Comparisons are made using two mean-field equations of state, one based on a Clausius hard-sphere reference fluid and the other based on a Carnahan/Starling hard-sphere reference fluid. The simplest and oldest of the models, the generalized van der Waals model, becomes unphysical at high mean pore densities. The Carnahan/Starling version of Tarazona model agrees best overall with the simulations. This model represents a systematic improvement on the generalized van der Waals model and is computationally the most complicated of all models examined. The YBG and generalized hard-rod models are not as accurate as the Tarazona model, but they capture the qualitative trends observed in the simulations. Both of these models are intuitive extensions of the exact theory of one-dimensional hard rods, and are computationally much simpler than the Tarazona model.